The invention relates to a mechanical seal, and more particularly, to a zero-leakage hydrodynamic face seal which prevents leakage of a fluid being sealed.
Mechanical face seals are used on various types of machines and equipment, such as pumps, compressors and gear boxes, which have rotating shafts and a sealing chamber adjacent the shaft wherein a fluid in the sealing chamber is prevented from leaking therefrom. Such mechanical seals include a pair of adjacent seal rings having opposing seal faces which define a sealing region therebetween. One of these seal rings typically is mounted on the shaft so as to rotate therewith while the other seal ring is non-rotatably mounted on a seal housing. The fluid being sealed is disposed on one edge of the sealing region, wherein the opposing seal faces at least reduce leakage of the sealed fluid across the sealing region.
Most liquid seals operate with the seal faces rotating in contact. However, due to asperities in the seal faces, some leakage may occur. In one example, a gearbox seal has used a carbon seal ring against a hard seal ring wherein the lubricity of the carbon permitted contacting operation. However, a continuing problem has been that the carbon face often blisters when used with viscous oils wherein the raised blisters cause leakage. Many unsuccessful attempts to resolve this problem have been attempted.
In gas seals, the seal faces often are provided with grooves or recesses to generate hydrodynamic lifting forces. In this regard, the sealing faces typically separate a small distance wherein a thin film of fluid forms between the seal faces to lubricate the seal faces and reduce wear therebetween. Additionally, the grooves or recesses may pump the fluid film toward the fluid being sealed to reduce leakage of the sealed fluid.
With respect to the specific constructions of mechanical seals, a wide variety of mechanical seals have been designed and used to varying degrees of success. One mechanical seal is disclosed in U.S. Pat. No. 4,491,331 (Salant), the disclosure of which is incorporated herein in its entirety. In this mechanical seal which is used in a submersible pump, two opposed seal faces rotate in sliding contact with each other with a thin fluid film being defined between the seal faces. The seal faces define a seal which sealingly separates internal lubricant from external well fluid. A plurality of concentric annular grooves are provided on one seal face, wherein due to surface tension forces, the liquid film is prevented from leaking into the grooves.
However, in this seal, the fluid film forms hydrostatically. Accordingly, a low balance ratio was used which permits formation of the fluid film but results in a seal which can be unstable during operation.
In view of the foregoing, it is an object of the invention to overcome disadvantages associated with prior art mechanical seals and particularly the seal of U.S. Pat. No. 4,491,331.
The invention relates to a hydrodynamic face seal having a pair of coaxially aligned seal rings which are relatively rotatable during rotation of a rotating shaft. The seal rings have opposing seal faces that define a sealing region therebetween which prevents leakage of a fluid along the shaft.
At least one of the seal faces includes a plurality of concentric shallow annular grooves which preferably are disposed near the seal ring diameter that is farthest away from the fluid being sealed. The shallow grooves are separated one from the other by narrow lands which extend radially therebetween, wherein the grooves have a rectangular, triangular or curved cross-sectional profile.
The opposing seal faces preferably are flat and substantially parallel at least in the annular region having the concentric shallow grooves. Liquid is supplied to the seal faces such that, during shaft rotation, a thin film of liquid forms between the seal faces. More particularly, in the region of the concentric grooves, gases separate from the liquid and collect within the concentric grooves while the fluid film still forms in the lands due to surface tension. As a result, alternating annular bands or rings of liquid and gas are formed wherein each adjacent pair of a liquid ring and a gas ring are separated from each other by a liquid-gas interface.
It is found that each band of gas and each adjacent band of liquid are at different pressures and define a pressure drop therebetween. As such, each liquid/gas interface supports a pressure drop whereby the plurality of concentric liquid/gas interfaces effectively prevent leakage of the film liquid across the seal faces. In this regard, the last concentric groove which is farthest from the fluid being sealed has the lowest pressure which pressure is sufficiently low such that the surface tension forces acting on the liquid film in the region of this last concentric groove prevent the liquid from exiting the seal faces.
In addition to the concentric grooves, the opposing seal faces also are provided with hydrodynamic lift features radially next to the grooves. In particular, the hydrodynamic lift features are provided in the annular region that is defined radially between the concentric grooves and the edge of the sealing region that is exposed to the fluid being sealed. Examples of hydrodynamic lift features include wavy faces, slots, T-grooves and spiral grooves.
The hydrodynamic lift features provide a hydrodynamic lift which provides a stable separation of the seal faces that permits formation of the fluid film between the seal faces. Rather than the fluid film being generated solely by the static pressure of the fluid being sealed, the hydrodynamic lift features generate lift dynamically during shaft rotation to provide a lift load which is stable.
Preferably, the hydrodynamic lift features are wavy faces which do not pump the film fluid radially toward the concentric grooves but rather act circumferentially on the film liquid. However, it also will be understood that other hydrodynamic lift features may be used such as those features identified above. As discussed in further detail herein, the use of hydrodynamic lift features to generate hydrodynamic lift in the seal in combination with the use of concentric grooves to provide the primary sealing function of the seal results in a seal which is effectively non-leaking and stable.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.